Turbulent shear force microsensor

Abstract

A microbridge is used for the accurate measuring of time varying shear forces in the presence of fluctuating pressure. A microdimensioned plate is suspended by arms to form a microbridge. The microdimensions enable the smallest turbulence scales of interest to be sensed uniformally throughout the entire surface of the plate. The cavity beneath the microbridge is so small that a viscous drag is created in the air within the cavity and dampens normal movement of the plate. The microdimensions in conjunction with the damping effect of the cavity enable the sensor to be substantially insensitive to pressure and thus sense lateral forces independent of normal forces. The microbridge sensor is fabricated by surface micromachining. A sacrificial layer is deposited over a substrate. A structural layer is deposited and patterned to form the plate and support arms over the sacrificial layer. The cavity is formed by a selective etchant removing the sacrificial layer and leaving the rest of the microbridge structure suspended above the substrate. In a differential capacitance readout scheme, a conducting layer in the plate of the microbridge is capacitively coupled with conductors in the substrate. A sensed change in capacitive coupling generates an indication of plate deflection and thereby shear stress independent of vertical movement. Optic readout schemes may also be employed and are readily incorporated in the fabrication process. A mounting member presses the microbridge sensor into a holding plate which fits in a matching slot flush with the target wall.

We claim: 
     
       1. A wall shear stress microsensor comprising: a microndiminsioned shear-sensitive plate;   a plurality of supporting arms which suspend the plate above a substrate and define a cavity between the plate and the substrate, the cavity being so small that it dampens movement of the plate normal to the plane of the plate through viscous forces generated within the cavity; and   readout means for providing an indication of the deflection of the plate and thereby a measurement of the shear stress of interest.   
     
     
       2. A wall shear stress microsensor as claimed in Claim 1 wherein the microndimensions of the plate cause environmental pressures to be uniformly felt throughout an entire top surface of the plate such that the plate in combination with the damping effect of the cavity renders the sensor substantially insensitive to pressures, and shear lateral forces are sensed substantially independent of pressure. 
     
     
       3. A wall shear stress microsensor as claimed in claim 2 wherein the microndimensions of the plate include a top surface area which is greater than an end surface area lying in a plane perpendicular to the plane in which the top surface area lies, and said top surface area is an order of magnitude smaller than an area in which the smallest eddy of interest is uniform. 
     
     
       4. A wall shear stress microsensor as claimed in claim 2 wherein the microndimensions of the plate include a dimension along a length of the top surface no bigger than about 5L* to about 10L* and a total height or protrusion above a target surface no higher than about 2L* to about 3L* where L* is a relevant length scale in turbulent flow such that   L*=ν/u*     where ν is a kinematic viscosity equal to μ/ρ and u* is friction velocity equal to ##EQU6## where μ is absolute viscosity of the target fluid, ρ is density and τ is shear stress.   
     
     
       5. A wall shear stress microsensor as claimed in claim 2 wherein the plate has a lateral dimension along a length of its top surface of about 1000 microns or less and is suspended about 10 microns or less above the substrate. 
     
     
       6. A wall shear stress microsensor as claimed in claim 5 wherein a longitudinal dimension of the cavity between the plate and substrate is less than a longitudinal dimension of the supporting arms between the substrate and a top surface of the supporting arms. 
     
     
       7. A wall shear stress microsensor as claimed in claim 1 wherein the readout means for providing an indication of the deflection of the plate and measurement of the shear stress includes an integrated differential capacitance circuit. 
     
     
       8. A wall shear stress microsensor as claimed in claim 7 wherein the integrated differential capacitance circuit comprises: a conducting layer attached to the plate;   three conductors attached to the substrate, two positioned to the outside of the third, the third conductor being in the middle, the conducting layer of the plate being aligned within the two outer positioned substrate conductors, the conducting layer being capacitively coupled with the conductors attached to the substrate; and   means for sensing a change in capacitive coupling between the conductive layer and the outer positioned substrate conductors where lateral motion of the plate changes the capacitive coupling between the conducting layer and the outer positioned substrate conductors, one outer positioned substrate conductor experiencing a lowered coupling, the other outer positioned substrate conductor experiencing an increased coupling, said sensing means sensing the change in coupling and producing a signal indicative of the deflection of the plate and thereby sensed shear stress.   
     
     
       9. A wall shear stress microsensor as claimed in claim 8 wherein the means for sensing a change in capacitive coupling includes: two sensing nodes, one for each outer positioned substrate conductor, each sensing node sensing the change in coupling experienced by its respective substrate conductor; and   two field effect transistors (FET's) one for each sensing node, where each sensing node senses a change in coupling of its respective outer positioned substrate conductor and places charge on its respective FET, the FET's connected to a differential amplifier circuit which is sensitive to the difference in current from the two FET's and therefrom provides an output signal indicative of the sensed wall shear.   
     
     
       10. A wall shear stress microsensor as claimed in claim 9 wherein the two FET's are essentially the same. 
     
     
       11. A wall shear stress microsensor as claimed in claim 9 wherein the differential amplifier circuit is off chip and comprises two amplifiers, one connected to one FET, the second connected to the other FET, the FET's changing current flow to each respective amplifier such that said amplifiers produce signals which are differentially compared to provide an output signal indicative of sensed wall shear. 
     
     
       12. A wall shear stress microsensor as claimed in claim 8 wherein said circuit is integrated into the microsensor by micro-fabrication of films on the substrate by the process of: depositing a sacrificial film on the substrate, the substrate fabricated with the conductors of the integrated circuit;   depositing a second film onto the sacrificial film such that said second film is connected to the conducting layer and patterned to form the plate and supporting arms; and   removing only the sacrificial film leaving the plate defined by the second film connected to the conducting layer and suspended above the substrate by the supporting arms, and leaving the conductors connected to the substrate.   
     
     
       13. A wall shear stress microsensor as claimed in claim 8 wherein an array of shear-sensitive plates with said support arms and said integrated differential capacitance circuits is repeated on one substrate to provide multi-point measurements. 
     
     
       14. A wall shear stress microsensor as claimed in claim 1 wherein the readout means includes optical readout means. 
     
     
       15. A wall shear stress microsensor as claimed in claim 14 wherein said optical readout means include: a window in the substrate;   a distinguishing optical feature associated with the plate and open to the window; where the distinguishing optical feature moves with the plate and is viewed through the window to provide an indication of the amount of plate deflection and thereby shear stress.   
     
     
       16. A wall shear stress microsensor as claimed in claim 15 wherein said optical readout means further include an optical scale associated with the window, said scale enabling a user to measure the amount of deflection of the plate and thereby shear stress by viewing through the window the change in position of the optical distinguishing feature of the plate relative to said scale where the optical distinguishing feature moves with the plate and the scale remains relatively stationary. 
     
     
       17. A wall shear stress microsensor as claimed in claim 1 wherein the plate is suspended within a recess in the substrate and the cavity is formed between the plate and base of the recess. 
     
     
       18. A wall shear stress microsensor as claimed in claim 17 wherein the plate has a lateral top surface which is flush with the surface of the substrate, said plate top surface being open to the turbulent flow creating the shear stress of interest. 
     
     
       19. A wall shear stress microsensor as claimed in claim 18 wherein said plate has lateral dimension along a length of the top surface which is n order of magnitude smaller than a length dimension of the smallest eddy of interest. 
     
     
       20. A wall shear stress microsensor as claimed in claim 18 wherein the microndimensions of the plate include a length dimension no bigger than about 5L* to about 10L* and a total height or protrusion above a target surface no higher than about 2L* to about 3L* where L* is a relevant length scale in turbulent flow such that ##EQU7## where ν is kinematic viscosity equal to μ/ρ and u* is friction velocity equal to ##EQU8## where μ is absolute viscosity of the target fluid, ρ is density and τ is shear stress. 
     
     
       21. A wall shear stress microsensor as claimed in claim 18 wherein said plate has a lateral dimension along a length of its top surface of less than about 1000 microns and the cavity is less than about 10 microns in height between the plate and base of the recess. 
     
     
       22. A wall shear stress microsensor as claimed in claim 17 wherein the readout means includes an integrated differential capacitance circuit. 
     
     
       23. A wall shear stress microsensor as claimed in claim 22 wherein the integrated differential capacitance circuit comprises: a conducting layer connected to the plate;   three conductors connected to the substrate, two of the three conductors positioned on the outside of the third, the third being in the middle, said conducting layer of the plate being aligned within the two outer positioned substrate conductors, the conducting layer being capacitively coupled with the conductors connected to the substrate; and   means for sensing a change in capacitive coupling between the conducting layer and the two outer positioned substrate conductors where lateral motion of the plate changes the capacitive coupling between the conducting layer and the outer positioned substrate conductors, one outer positioned substrate conductor experiencing a lowered coupling, the other outer positioned substrate conductor experiencing an increased coupling, said sensing means sensing the change in coupling and producing a signal indicative of the deflection of the plate and thereby sensed shear stress.   
     
     
       24. A wall shear stress microsensor as claimed in claim 23 wherein the means for sensing a change in capacitive coupling includes: two sensing nodes, one for each outer positioned substrate conductor, each sensing node sensing the change in coupling experienced by its respective substrate conductor;   two field effect transistors (FET's) one for each sensing node where each sensing node senses a change in coupling of its respective outer positioned substrate conductor and places charge on its respective FET, the FET's connected to a differential amplifier circuit which is sensitive to the difference in current flowing from the two FET's and therefrom provides an output signal indicative of the sensed wall shear.   
     
     
       25. A wall shear stress microsensor as claimed in claim 24 wherein the two FET's are substantially the same. 
     
     
       26. A wall shear stress microsensor as claimed in claim 24 wherein the differential amplifier circuit is off-chip and comprises two amplifiers, one connected to one FET, the second connected to the other FET, the FET's changing current flow to their respective amplifier such that said amplifiers produce signals which are differentially compared to provide an output signal indicative of sensed wall shear. 
     
     
       27. A wall shear stress microsensor as claimed in claim 23 wherein said circuit is integrated into the microprocessor by microfabrication of films in the recess of the substrate by the process of: depositing a sacrificial film in the substrate recess, the base of the recess fabricated with the three conductors of the circuit;   depositing a second film over the sacrificial film such that said second film is connected to the conducting layer and is patterned to form the plate and supporting arms; and   removing only the sacrificial film and   leaving the plate defined by the second film connected to the conducting layer and suspended within the recess by the support arms, and the conductors connected to the base of the recess.   
     
     
       28. A wall shear stress microsensor as claimed in claim 22 wherein an array of shear-sensitive plates with said arms and said integrated differential capacitance circuits is repeated on one substrate to provide multiple point measurements. 
     
     
       29. A wall shear stress microsensor as claimed in claim 17 wherein the readout means includes optical readout means. 
     
     
       30. A wall shear stress microsensor as claimed in claim 29 wherein said optical readout means include: a window in the base of the recess; and   a distinguishing optical feature on the plate open to the window where the distinguishing optical feature moves with the plate and is viewed through the window to provide an indication of the amount of plate deflection and thereby the amount of detected shear stress.   
     
     
       31. A wall shear stress microsensor as claimed in claim 30 wherein said optical readout means further include an optical scale associated with the window, said scale enabling a measurement of the amount of deflection of the plate and thereby shear stress to be obtained by viewing through the window the change in position of the distinguishing optical feature of the plate relative to said scale where the distinguishing optical feature moves with the plate and the scale remains relatively stationary. 
     
     
       32. A wall shear stress microsensor as claimed in claim 1 further comprising mounting means to position said shear-sensitive plate, arms and readout means in a target flow substantially flush with a target wall such that said target flow is not disturbed by said plate, arms and readout means. 
     
     
       33. A wall shear stress microsensor as claimed in claim 32 wherein said mounting means includes a holding plate into which are evenly and smoothly fitted one or more substrates on which the shear-sensitive plate, arms and readout means are fabricated, where said holding plate fits into a matching slot and is adhered in place in a support structure in the target wall which covers a large surface area of the target wall compared to the size of a substrate and which is smooth with the target wall. 
     
     
       34. A sheer stress sensor having a bridge shape of microndimensions which render it substantially insensitive to pressure and sensitive to lateral forces independent of pressure, by the microndimensions causing the effects of pressure to be uniform throughout both an entire top surface and a bottom surface of the sensor and the microndimensions defining a passageway under the sensor so small that a damping effect of geometrically normal movement of the sensor under effects of vibrational forces is produced by a viscous drag within that passageway. 
     
     
       35. A shear stress sensor as claimed in claim 34 wherein the microdimensions are an order of magnitude less than dimensions of the smallest eddies of interest. 
     
     
       36. A shear stress sensor as claimed in claim 34 wherein the microndimensions include a dimension along a length of the top surface no larger than about 5L* to about 10L* and protrusion above a target surface no more than about 2L* to about 3L* where L* is a relevant length scale in turbulent flow defined by   L*=ν/u*     where ν is kinematic viscosity equal to μ/ρ and u* is frictional velocity equal to ##EQU9## where μ is absolute viscosity of the target fluid, ρ is density and τ is shear stress.   
     
     
       37. A shear stress sensor as claimed in claim 34 wherein said dimensions include: a dimension along a length of the top surface less that about 1000 microns, and a height under the sensor of less than about 10 microns. 
     
     
       38. A shear stress sensor as claimed in claim 34 wherein said microndimensions are generated by microfabrication of two films on a substrate by the process of: depositing a sacrificial film on the substrate;   depositing onto the sacrificial film a second film and patterning said second film to form a microndimensioned top surface; and   removing the sacrificial film leaving the microdimensioned top surface defined by the second film suspended above the substrate at the very small height and leaving a cavity where the sacrificial film was previously deposited.   
     
     
       39. A shear stress sensor as claimed in claim 38 wherein said second film is further deposited over the whole surface of the substrate leaving an undeposited area for access to the sacrificial film on one side of the defined top surface and leaving another area for allowance for lateral deflection on an opposite side of the top surface, said further deposited second film being made to a thickness which is greater than the height under the sensor. 
     
     
       40. A bridge-shaped structure having a top surface exposed to environmental effects and an opposite surface facing a plane on which legs of the bridge structure stand, the bridge structure being of micron dimensions which render it substantially insensitive to pressure and sensitive to lateral forces independent of pressure, by the micron dimensions causing the effects of pressure to be uniform throughout the entire top surface of the bridge-shaped structure and the micron dimensions defining a space between the opposite surface and the plane under the bridge structure so small that a damping effect of normal movement of the bridge-shaped structure is produced by a viscous drag within the space. 
     
     
       41. A bridge structure as claimed in Claim 40 wherein the micron dimensions are an order of magnitude less than smallest eddies to which the bridge structure is exposed. 
     
     
       42. A bridge structure as claimed in claim 40 wherein the micron dimensions include a lateral dimension of the top surface no larger than about 5L* to about 10L* and a protrusion above a target surface no more than about 2L* to about 3L* where L* is a relevant length scale in turbulent flow defined by   L*=ν/u*     where ν is kinematic viscosity equal to μ/ρ and u* is friction velocity equal to ##EQU10## where μ is absolute viscosity of the target fluid, ρ is density and τ is shear stress.   
     
     
       43. A bridge structure as claimed in claim 58 wherein said dimensions include: a top surface lateral dimension less than about 1000 microns and a height under the bridge structure of less than about 10 microns.   
     
     
       44. A bridge structure as claimed in claim 40 wherein said micron dimensions are generated by microfabrication of two films on a substrate by the process of: depositing a sacrificial film on the substrate;   depositing onto the sacrificial film a second film and patterning and said film to form a micron dimensioned top surface; and   removing the sacrificial film leaving the micron dimensioned top surface defined by the second film suspended above the substrate at the very small height and leaving a cavity where the sacrificial film was previously deposited.   
     
     
       45. A bridge structure as claimed in claim 44 wherein said second film is further deposited over the whole surface of the substrate leaving an undeposited area for access to the sacrificial film on one side of the defined top surface and leaving another area for allowance for lateral deflection on an opposite side of the top surface, said further deposited second film being made to a thickness which is greater than the height under the bridge structure. 
     
     
       46. A micron dimensioned sensor for sensing lateral movement, the sensor comprising: a micron dimensioned plate;   a plurality of supporting arms which suspend the plate above a substrate and define a cavity between the plate and the substrate, such that the plate is sensitive to lateral forces independent of pressure by the cavity being so small that it dampens movement of the plate normal to the plane of the plate through viscous forces generated within the cavity; and   means for providing an indication of deflection of the plate and thereby a measurement of lateral movement.   
     
     
       47. A micron dimensioned sensor as claimed in claim 46 wherein the micron dimensions of the plate cause environmental pressures to be uniformly felt throughout an entire top surface of the plate such that the plate in combination with the damping effect of the cavity renders the sensor substantially insensitive to pressures, and lateral forces are sensed substantially independent of pressure. 
     
     
       48. A micron dimensioned sensor as claimed in claim 46 wherein the micron dimensions of the plate include a top surface area which is greater than an end surface area lying in a plane perpendicular to the plane in which the top surface area lies, and said top surface area if an order of magnitude smaller than an area in which the smallest eddy of interest is uniform. 
     
     
       49. A micron dimensioned sensor as claimed in claim 46 wherein the plate has a lateral dimension along a length of its top surface of about 1000 microns or less and is suspended about 10 microns or less above the substrate. 
     
     
       50. A micron dimensioned sensor as claimed in claim 46 wherein the means for providing an indicating of deflection provides a measurement of acceleration. 
     
     
       51. A shear stress microsensor comprising: a plate having one surface open to shear force of interest and a surface opposite the one surface, the one surface having a lateral dimension of less than about 1000 microns such that the smallest eddies of interest are uniformly felt across the one surface of the plate;   four microndimensioned support arms, one end of each arm connected to the plate, an opposite end of each arm connected to a substrate such that said arms suspend the plate above the substrate at a height of less than about 10 microns with said plate surface opposite the one surface facing the substrate where a cavity is formed there between, the cavity dampening movement of the plate normal to the substrate by generating a viscous drug;   a conducting layer connected to the plate and capacitively coupled with three conductors attached to the substrate, two conductors positioned one on one side of the third conductor and the second on an opposite side of the third conductor, the third conductor connected to generator means, the conducting layer aligned between longitudinal axes along which the two conductors are respectively positioned on the opposite sides of the third conductor; and   means for sensing a change in capacitive coupling of the conducting layer and the three conductors, said change in coupling being indicative of lateral deflection of the plate and thereby sensed shear stress, the lateral deflection being detected substantially independent of pressure affects and movement of the plate normal to the substrate.   
     
     
       52. A shear-stress microsensor as claimed in claim 51 wherein said means for sensing a change in capacitive coupling includes to matching on-chip field effect transistors, one transistor coupled to a first sensing node for sensing change in capacitive coupling of the conducting layer and one conductor, the second transistor coupled to a second sensing node for sensing change in capacitive coupling of the conducting layer and another conductor. 
     
     
       53. A shear-stress microsensor as claimed in claim 51 further comprising mounting means to position said plate, arms and means for sensing a change in capacitive coupling, in a target flow substantially flush with a target wall such that said target flow is not disturbed by said plate, arms and readout means. 
     
     
       54. A shear-stress microsensor as claimed in claim 53 wherein said mounting means includes a holding plate into which are perfectly fitted one or more wafers on which the plate, arms and sensing means are fabricated, where said holding plate fits into a matching slot and is adhered in place in a support structure in the target wall which covers a large surface area of the target wall compared to the size of a wafer and which is smooth with the target wall.